Growing up in North miami beach, Tobi Cohen Kosanke, now 48, adored her brother Keith. Seven years older, he was a “laid-back surfer dude,” while she was a “chubby, nerdy” little girl. Tobi knew she could never live up to Keith’s cool persona, so while he was quitting school, experimenting with drugs, and focusing on riding the next wave, Tobi threw herself into school, with her brother’s encouragement. The hard work paid off: She went on to earn her Ph.D. and become a geologist. “I hung out with the geeky kids, the good kids, the smart kids, because of my brother,” she says. “I loved Keith, and I know he was proud of me, but I owe my success to taking the road that he didn’t take.”

Tobi’s story is not unusual. Of all the factors that shape your personality—your genes, your parents, your peers—siblings are at the top, according to one major theory of human development. If you think about it, the relationships with your sisters and brothers will likely last longer than any others in your lifetime. Research shows that even in adolescence, you spend 10 to 17 hours a week with them—and experts are finding that their impact continues long after you’ve left the nest. Study after study has shown that the ways you interact with each other growing up can affect your relationships, your happiness, even the way you see yourself throughout the rest of your life.

For this story, Matt Groening flashed forward to give us a peek at the Simpson siblings as grown-ups. (Illustration by Julius Preite)

“I’m first!” “I’m the baby!”Some of the earliest studies of siblings focused on how birth order influences personality and fate. You’re familiar with the basic types: Firstborns are said to be responsible and high-achieving, youngest siblings charmers and free spirits, and middle children lost in the mix.

It’s easy to dismiss these as mere stereotypes, and indeed there are researchers who do, but others have found statistical evidence that bears them out. A Norwegian study found that firstborns had slightly higher IQs than their sibs. Other research has shown they’re also more successful: According to Sandra Black, Ph.D., professor of economics at the University of Texas at Austin, “Firstborns earn more than secondborns, who earn more than thirdborns.”

On the other hand, research has found that youngest siblings really do tend to be risk-takers. Frank Sulloway, Ph.D., of the Institute of Personality and Social Research at UC Berkeley, studied baseball-playing brothers—like Joe, Dom, and Vince DiMaggio—and found that the younger ones tried to steal base more often than their older brothers. Meanwhile, middle children grow up to be more peer-oriented, says Sulloway. First- and last-borns turn to parents in an emotional crisis; middle kids, to their friends. Still, birth order is hardly destiny, says Sulloway. What’s more important, researchers say, is the quality and dynamics of your relationships with your siblings.

“I’m nothing like him!”Within a family, children devise all sorts of strategies to increase their status and feeling of belonging, and one of the most important is what experts call “sibling de-identification.” To reduce competition with brothers and sisters who may be cuter or smarter (not to mention bigger and stronger), we each carve out our own niche.

Much like Tobi Kosanke, younger siblings typically start out adoring their older brothers or sisters, says Laurie Kramer, Ph.D., professor of applied family studies at the University of Illinois at Urbana. “They want to mimic their strengths and talents, but over time, they realize they can’t succeed at the same level. That leads them to develop their own attributes.”

“You can be the smartest kid in your class or the fastest on the track team,” says Susan McHale, Ph.D., professor of human development at Penn State University. “But if you have a brother or sister who’s smarter or more athletic, it doesn’t matter.” In other words, your self-image is shaped at least in part by how you compare to your siblings.

And it’s not just that younger kids de-identify from more capable older siblings, says McHale. In early adolescence, when we’re trying to figure out who we are, it’s often older siblings who emphasize their differences. For example, a boy with a feminist younger sister might adopt a more macho stance. Even among tight-knit sibs, “you want to be close but also to be your own person,” says Victoria Hilkevitch Bedford, Ph.D., professor emerita of psychology at the University of Indianapolis.

And experts speculate that our tendency to compare ourselves to our siblings continues well into adulthood. For example, the sibling dynamic could affect what we try to achieve, says Kramer. “Asked to give a speech or do a challenging job, a less accomplished younger sibling might decline, thinking, ‘If they knew my older brother, they wouldn’t think I was so great.’ ”

“Oh, her? She’s just one of the guys.”When it comes to learning about the opposite sex, researchers say, there’s nothing better than having an older member at home. “If you are a girl with an older brother or a boy with an older sister, you should thank them for whatever romantic success you’ve had,” jokes William Ickes, Ph.D., professor of psychology at the University of Texas at Arlington.

In Ickes’s now classic 1983 study, he instructed unacquainted male-female pairs to talk to each other. Girls with older brothers and boys with older sisters broke the ice more easily and were more likely to rate each other favorably.

Because the genders don’t mix much in middle childhood, “kids who see opposite-sex siblings and their friends in everyday settings may come to know more about how the other sex behaves and connects,” says McHale.

Melissa Payne, a 29-year-old medical industry account manager in Orlando, says her relationship with her 33-year-old brother, Dave Payne, a publicist in Tarpon Springs, Fla., not only helped pave the way for romantic connections but allowed the two to share dating advice. A few years ago, when both siblings were seeing people who did not treat them well, each reminded the other that not all women—or men—were such shabby partners. Within a year, they had broken off the relationships. “When you’re dating someone, you can make excuses, but it was different hearing a guy’s perspective from my brother,” says Melissa.

“Hey, that’s mine!”Young siblings fight up to eight times an hour, research shows. While all that squabbling may drive parents crazy, it’s also how some kids learn to negotiate conflict—training ground for dealing with neighbors, bosses, and spouses down the line.

When it comes to arguing and expressing our opinions, we can take risks with our siblings because they’re stuck with us, says Corinna Jenkins Tucker, Ph.D., associate professor of family studies at the University of New Hampshire. “Children can test which conflict resolution strategies work and which don’t,” she says. (Refuse to share? Just watch your brother’s reaction.) And what we learn during childhood can have far-reaching effects.

Kids who learn coercive or hostile approaches to handling conflict are more likely in adolescence to join risky peer groups and engage in negative social behaviors (like smoking, drinking, or skipping school), according to McHale. But siblings can be taught to compromise—that’s why parental involvement is so crucial. Reluctant referees who consider sibling aggression normal and don’t help resolve clashes are making a mistake, says McHale. Their kids may end up with poorer social skills and more conflict compared with kids whose parents help them work out their disagreements.

And what about as you grow older? Some experts say that kids who learn hostile patterns of interaction with sibs may repeat those patterns with friends or coworkers. Others, however, suggest that kids who never develop close relationships with their brothers or sisters may be more likely to go out of their way to form strong connections outside the family.

“Of course we love you both the same.”It’s not possible to talk about the sibling relationship without considering Mom and Dad, the central pole on the family merry-go-round. Siblings may receive a lot of things from their parents, including cues on how to treat someone in a close relationship. Good marriages tend to make for kids who get along better, says Katherine Jewsbury Conger, Ph.D., associate professor of human development at UC Davis.

Yet there’s one parental behavior that can really make or break the sibling relationship. As every kid knows almost from his or her first breath, if Mommy or Daddy gives me less than my sister, then it’s game over. Social scientists call this “differential treatment.” Kids call it favoritism, and if we think Susie is Mommy’s favorite, we don’t like it, and sometimes maybe don’t like Susie, either.

Children of different ages and abilities are bound to be treated differently, says Conger, but for kids, the real question is fairness. And if children see differential treatment as unfair, those negative feelings can last—even into the next generation, with an adult sibling resenting that Grandma gives Susie’s kids better Christmas gifts.

Paige D. feels that favoritism came between her and her older brother, with whom she no longer speaks (she says their parents skipped both her college graduation and her wedding but attended her brother’s graduation). “As a child, I loved my brother more than the moon and the stars,” says Paige. “But I think the overt favoritism made him feel so uncomfortable that it was easier for him to block me out as a way to justify our parents’ eccentricity.”

Interestingly, though, while some children are highly attuned to variations in their parents’ attentions, they are often mistaken about favoritism. When Deborah T. Gold, Ph.D., associate professor of medical sociology at Duke University, studied pairs of adult siblings, she found that in many cases each thought the other was their parents’ favorite.

“What are we going to do about Mom?”How siblings get along in adulthood also depends greatly on how they manage one of the most volatile family passages—the aging and death of their parents. As grown-up sibs see their time with Mom or Dad running out, it stirs up deep childhood desires for love and approval. Research shows that in 90 percent of families, one person does most of the caregiving, and Bedford notes that if siblings grew up with a sense of unfairness, those feelings can reignite over how sisters and brothers perceive elder care. On the other hand, this passage also brings enormous opportunities to strengthen and renew sibling relationships. Putting your heads together during stressful times can help you and your siblings get to know each other as adults. Hey, my brother’s not that incompetent little kid! Where did my sister acquire so much patience?

And even if things are tense (or worse), it’s not impossible to repair the relationship. As young adults, Wendy Beckman and her older sister, Bonnie Nielson, were little more than cordial. Wendy, now 55 and a writer in Cincinnati, still thought of her older sister as annoyingly overprotective. But in her 30s, she made efforts to connect with Bonnie, whose marriage was unraveling.

“Bit by bit,” Wendy recalls, “we started talking honestly about things in our adult lives, not just ‘You took my socks when I was 12.’ She actually started asking me for advice!”

Confronting their father’s Alzheimer’s and their mother’s death from cancer, they grew even closer. “We still come at the world completely differently,” says Bonnie, now 61, of Augusta, Maine. “But when you see your siblings as fully formed adults, the relationship is so much more fulfilling.”

Experts say this pattern of sibling drift, followed by reconnection, is common. When siblings move away and start their own careers and families, they often have little contact except through their parents. But in middle age and beyond, as other loved ones pass away, surviving siblings can be important sources of support. In fact, research shows the healthiest, happiest, and least lonely people have warm sibling relationships.

As time passes, Patti Wood’s relationship with her two older sisters has become more precious. Patti, 53, an author and speaker in Decatur, Ga., always adored her sisters, now 62 and 66. The three “military brats” stayed close during their many moves, and have bonded even more tightly after caring for their 92-year-old mother.

Despite their differences—single, long-married with grandkids, divorced with a grown child—they talk to each other nearly every day, travel together, and call each other first in a crisis. And they share a unique history. Patti’s oldest sister, Robin, speaks for them all when she says, “We know we’ll always be there for each other. I can’t imagine not having my sisters there to count on.”

Only the lonely?Pity the poor only child—no one to play with. (Or maybe you envied onlies, with no bratty little brother trashing their stuff.) Regardless, “there’s a big misconception in American popular culture that singletons are selfish, lonely, or maladjusted,” says Toni Falbo, Ph.D., a professor of educational psychology at the University of Texas at Austin and an only child herself. But in fact, only children have one major advantage, says Falbo: They don’t have siblings competing for their parents’ resources, including college funds. As a result, onlies tend to achieve high levels of education and occupational prestige.

As little kids, Falbo acknowledges, single children may be more comfortable with adults. “But peer sociability grows with experience,” she says, and by high school, onlies are on par with kids who have sisters and brothers. “Every day, my mother shoved me out the door and insisted I play with other children, which forced me to develop social skills,” says Falbo. “And it also helped me learn to appreciate my family, which was just the right size for me.”